Several methods are available for the production of images viewable by transmitted light. While the resulting imaged transparencies find utility for a number of applications, for certain purposes it has often been desired that the image be viewable as a reflection image. In consideration of the relative fragility of images on certain transparencies, it has also often been desired that such images be protected from damage and environmental stress. Despite the desire for both, the satisfactory unification of "reflective" and "protective" functionalities has been frustrated by their perceived incompatibility, the incompatibility being heightened in the context of predefined preparation, exposure, and development regimens oftentimes associated in the manufacture of imaged transparencies.
As a representative example, an imaged transparency is described in the embodiments disclosed in International Patent Application No. PCT/US87/03249 (Publication No. WO 88/04237) (Etzel), the disclosure being incorporated herein by reference. International Patent Application No. PCT/US87/03249 describes, in one embodiment, a thermal imaging medium and a process for forming an image in which a layer of a porous or particulate image-forming substance (preferably, a layer of carbon black) is deposited on a heat-activatable image-forming surface of a transparent first web material (hereinafter the "first transparent substrate"), the layer having a cohesive strength greater than its adhesive strength to the first sheet like element. Portions of this thermal imaging medium are then exposed to brief and intense radiation (for example, by laser scanning), to firmly attach exposed portions of the image-forming surface to the first transparent substrate. Finally, those portions of the image-forming substance not exposed to the radiation (and thus not firmly attached to the first transparent substrate) are removed, thus forming a binary image comprising a plurality of first areas where the image-forming substance is adhered to the first transparent substrate and a plurality of second areas where the first transparent substrate is free from the image-forming substance. Hereinafter, this type of image will be called a "differential adhesion" binary image. For the purposes of the present disclosure, such binary image may be considered an imaged transparency.
In a principal embodiment of the thermal imaging medium described in the aforementioned International Patent Application, the image-forming substance is covered with a second transparent substrate so that the image-forming substance is confined between the first and second transparent substrates. After imaging and separation of the unexposed portions of the image-forming substance (with the second transparent substrate) from the first transparent substrate, a pair of binary images each supported by a transparent substrate is obtained. A first binary image comprises exposed portions of image-forming substance more firmly attached to the first transparent substrate by heat activation of the heat-activatable image-forming surface. A second binary image comprises non-exposed portions of the image-forming substance carried or transferred to the second transparent substrate. For the purposes of the present disclosure, both binary images may be considered imaged transparencies.
The respective binary images obtained by separating the two transparent substrates of an exposed thermal imaging medium having an image-forming substance confined therebetween may exhibit substantially different characteristics. Apart from being the imagewise "positive" or "negative" of an original, the respective images may differ in character. Differences may depend upon the properties of the image-forming substance, on the presence of the original layer(s) in the medium, and upon the manner in which such layers fail adhesively or cohesively upon separation of the substrates. Either of the pair of images may, for reasons of informational content, aesthetic or otherwise, be desirably considered the principal image, and the invention described herein provides utility with regard to both types of images.
The image-forming process described in the aforementioned International Patent Application can produce high quality, high resolution imaged transparencies. However, for certain applications, the binary images produced on the transparent substrates by this process may suffer from comparatively low durability because, in the finished image, the porous or particulate image-forming substance, typically carbon black admixed with a binder, lies exposed on the surface of the transparent substrate, and may be smeared, damaged or removed by, for example, fingers or other skin surfaces (especially, if moist), solvents or friction during manual or other handling of the image.
Due to their relative fragility and/or environmental sensitivity, previous efforts have been directed toward the protection of such binary images with a protective coating or layer. In this regard, International Patent Application No. PCT/US91/08345 (published as WO 92/09930 on Jun. 11, 1992) (Fehervari, et al.), for example, describes a process for protecting a binary image by lamination thereto of a transparent overcoat. Likewise, U.S. Pat. No. 5,501,940 (Bloom, et al.) filed May 20, 1993 describes a process for protecting a binary image also involving lamination thereto of a transparent overcoat. With emphasis focussed on maintaining the transparent character of the underlying transparent binary image, neither process provides significant insight into converting an imaged transparency into a reflection image, the reflection image having comparable durability.
Converting an imaged transparency to a durable reflection image poses particular difficulties. First, as indicated above, binary images supported on a transparent substrate are often relatively fragile. Accordingly, especially for those transparencies developed and imaged to a high-resolution, heightened care must be exercised in ensuring that such resolution is not damaged by subsequent post-development conversion processes. Second, exposure of a thermal imaging medium, for example, to produce an imaged transparency typically requires irradiation through a transparent substrate or layer. Prior to such irradiation, incorporation of reflective pigments or a reflective layer may frustrate imagewise exposure; conversion by incorporating reflective pigments or layer would accordingly appear counterproductive. Third, regardless of when and where conversion is effected, compatibility with existing formats provides further constraints against the obvious incorporation of additional materials, such as reflective pigments, into an imaged transparency, pre-protected or otherwise. As a further complication, each of the enumerated difficulties is heightened by the desire to provide an image that is not only viewable by reflection, but one that also has durability comparable to the protected images described in the aforementioned International Patent Application No. PCT/US91/08345 and U.S. Pat. No. 5,501,940.
In light of the above, there is a need for means whereby an imaged transparency may be converted into an image capable of being viewed by reflected light, the resulting reflection image being durable. With regard to durability, the resulting reflection image should be, for example, suitable for archival purposes, abrasion-resistant, permit repeated solvent washings without risk of separating the durable layer from the underlying imaged transparency, and capable of maintaining the unitary integrity of the reflection image when cut into smaller sheets.